The dominant commercial process for the manufacture of adipic acid involves the air-oxidation of cyclohexane to form a mixture of cyclohexanol and cyclohexanone, which is subsequently oxidized by nitric acid to form a mixture of carboxylic acids, including adipic acid. Disadvantages of this process include low initial conversion of cyclohexane, formation of nitrated by-products, and generation of large wastewater and wastegas streams. Consequently, efforts have been made to find alternate routes to adipic acid.
Von Kutepow has disclosed the production of adipic acid by the reaction of butadiene, carbon monoxide and water at elevated temperature and pressure and in the presence of a halogen-promoted rhodium catalyst and an aromatic or saturated aliphatic solvent (U.S. Pat. No. 3,876,695). Yields of adipic acid range from 23% to about 50%.
Burke has disclosed a two-step route to adipic acid which comprises hydrocarboxylating butadiene to unsaturated C.sub.5 monocarboxylic acids in selected halocarbon solvents using an iodide-promoted rhodium catalyst, and then further hydrocarboxylating these C.sub.5 acids to adipic acid using the same catalyst system and solvent. The preferred temperature for the first step is 100.degree.-60.degree. C., while that of the second step is 150.degree.-180.degree. C. (U.S. Pat. No. 4,622,423 and U.S. Pat. No. 4,788,333). Although the linear selectivity of these reactions is high, and the over-all yield of adipic acid is substantially over 50%, the rates of reaction, especially that of the first step, are relatively low.
Burke has disclosed that the rhodium-catalyzed, iodide-promoted, hydrocarboxylation of certain unsaturated esters and terminally unsaturated alkenes can be accelerated by the addition of aliphatic or aromatic acids, having a pKa in the range of 4.2 to 5.2 (U.S. Pat. No. 4,788,334). However, the use of such accelerators results in a decrease in the linearity of the hydrocarboxylated products.
Burke has disclosed the interconversion of linear and branched saturated alkyl carboxylic acids by heating in the presence of an iodide or bromide promoted rhodium catalyst and carbon monoxide (U.S. Pat. No. 4,939,298).
The process of this invention seeks to overcome the disadvantages of the prior art by providing a high yield, high-rate, low-pollution process for the manufacture of adipic acid.